Spark plugs have been used for internal-combustion engines. For instance, a spark plug is utilized, which includes a cylindrical insulator having an axial bore extending in a direction of an axis, a metal shell disposed around an outer circumference of the insulator, and a center electrode disposed at the axial bore of the insulator.
A metal shell is manufactured by various processes such as forging and cutting. As a result of such manufacturing processes, the inner circumferential surface of the metal shell may take various forms. For instance, an unintended member (also called a foreign material) such as cutting chips may adhere to the inner circumferential surface of the metal shell. Such a foreign material can cause a defect. For instance, when a foreign material adheres to the inner circumferential surface of the metal shell, discharge may occur from the foreign material and not from an electrode.
The present description discloses a technique to appropriately determine whether or not the inner circumferential surface of a metal shell has a predetermined form.